New Atomic Structure Found In Metallic Glasses

2012-05-16Research and development

Researchers discovered a new nanometer-scale atomic structure in solid metallic materials known as metallic glasses, filling a gap in understanding of this atomic structure. In studies of a zirconium-copper-aluminum metallic glass, they found there are clusters of squares and hexagons — in addition to clusters of pentagons, some of which form chains — all located within the space of just a few nanometers.

Glasses lack a regular geometric arrangement of atoms over long distances

Researchers
have discovered a new nanometer-scale atomic structure in solid metallic
materials known as metallic glasses, filling a gap in understanding of this
atomic structure.

Glasses include all solid materials that have a
non-crystalline atomic structure. They lack a regular geometric arrangement of
atoms over long distances. "The
fundamental nature of a glass structure is that the organization of the atoms
is disordered—jumbled up like differently sized marbles in a jar, rather than
eggs in an egg carton," says Paul Voyles, a University of Wisconsin-Madison
associate professor of materials science and engineering and principal
investigator on the research.

Researchers widely believe that atoms in metallic glasses
are arranged only as pentagons in an order known as five-fold rotational
symmetry. However, in studies of a zirconium-copper-aluminum metallic glass,
they found there are clusters of squares and hexagons — in addition to clusters
of pentagons, some of which form chains — all located within the space of just
a few nanometers. "One or
two nanometers is a group of about 50 atoms—and it's how those 50 atoms are
arranged with respect to one another that's the new and interesting part,"
Voyles says.

This could
help manufacturers fine-tune such properties of metallic glasses as ductility,
the ability to change shape under force without breaking, and formability, the
ability to form a glass without crystalizing.

Measuring the atomic structure of glass at ...

Researchers
have discovered a new nanometer-scale atomic structure in solid metallic
materials known as metallic glasses, filling a gap in understanding of this
atomic structure.

Glasses include all solid materials that have a
non-crystalline atomic structure. They lack a regular geometric arrangement of
atoms over long distances. "The
fundamental nature of a glass structure is that the organization of the atoms
is disordered—jumbled up like differently sized marbles in a jar, rather than
eggs in an egg carton," says Paul Voyles, a University of Wisconsin-Madison
associate professor of materials science and engineering and principal
investigator on the research.

Researchers widely believe that atoms in metallic glasses
are arranged only as pentagons in an order known as five-fold rotational
symmetry. However, in studies of a zirconium-copper-aluminum metallic glass,
they found there are clusters of squares and hexagons — in addition to clusters
of pentagons, some of which form chains — all located within the space of just
a few nanometers. "One or
two nanometers is a group of about 50 atoms—and it's how those 50 atoms are
arranged with respect to one another that's the new and interesting part,"
Voyles says.

This could
help manufacturers fine-tune such properties of metallic glasses as ductility,
the ability to change shape under force without breaking, and formability, the
ability to form a glass without crystalizing.

Measuring the atomic structure of glass at this scale has
been extremely difficult. Researchers know that, at a few tenths of a
nanometer, atoms in metallic glasses have the same distances between them as
they do in crystals. They also know that at long distances—hundreds of
nanometers—there's no order left. "But what happens in between, at this 'magic' length of one to
three nanometers, is very hard to measure experimentally and is essentially
unexplored in experiments and simulations," says Voyles.

They used a scanning transmission electron microscope which
can generate an electron probe beam two nanometers in diameter — the ideal size
for examining atoms on a length scale of one to three nanometers. "And
that, fundamentally, is what makes the experiments work and gives us access to
this information that's otherwise very difficult to obtain," he says. "We can match our experimental
probe in size right to the size of what we want to measure."

They coupled the experimental data from the microscope with
a numerical model to conduct simulations that accurately reflect the
experiments. There were several clues in the properties of some metallic glasses
that these competing geometric structures might exist. Those arise from the
interrelationships of structure, processing and properties. "If we understand how the structure
controls, for example, glass-forming ability or the ability to change shape on
bending or pulling, and we understand how different elements participate in
these different kinds of structures, that gives us a handle on controlling
properties by adjusting the composition or adjusting the rate at which the
material was cooled or heated to change the structure in some useful way,"
Voyles says.

One of the unique characteristics of glasses is their
ability to transition continuously from a solid to a liquid state. While other materials, when heated,
are partly melted and partly solid, glasses as a whole become increasingly
malleable.

While manufacturers now apply metallic glasses primarily in
electrical transformer cores, their special forming capabilities may enable
manufacturers to make very small, intricate parts. "Unlike conventional metallic alloys,
metallic glasses can be molded like plastic—so they can be pushed or sucked or
blown into very complicated shapes without any loss of material or
machining," says Voyles.

Those manufacturing methods hold true even at the micro or nanoscale, so
it's possible to make, for example, forests of nanowires or the world's
smallest geared motor. The next step will be to calculate the properties of the
most realistic structural models of metallic glass they have developed to learn
how those properties relate to the structure.